1. Field of the Invention
The present invention relates broadly to a diaphragm valve wherein a flexible diaphragm cooperates with an associated valve seat to control a flow of a fluid. More specifically, this invention is directed to improvements in a diaphragm valve of the class wherein a flexible diaphragm is backed-up by a rigid spring retainer biased by a coiled spring.
2. Description of the Prior Art
Diaphragm valves are known, for example, from Japanese Utility Model Kokai Publication Nos. 53-35393 and 1-105713. Although some type of the conventional diaphragm valves, such as the ones shown in Japanese Utility Model Kokai Publication No. 62-41968, are designed such that a biasing force required to urge the diaphragm against an associated valve seat is developed by buckling of an intermediate portion of an elastomeric diaphragm, it has been the general practice in the art to use a coiled spring so as to bring the diaphragm in a sealing contact with the valve seat. The use of a separate coiled spring is preferable, since the diaphragm can be made more flexible. An example of such diaphragm valves having coiled springs is shown in FIG. 1.
In the diaphragm valve shown in FIG. 1, a fluid flow between an inlet 1 and an outlet 2 is controlled by a flexible diaphragm 3 which opens and closes a valve seat 4. The diaphragm is made of an elastomeric material and is backed-up by a spring retainer 5 of a rigid material such as metal or rigid plastics. A coiled spring 6 acting upon the spring retainer is arranged in a sensing chamber 7 defined by the diaphragm and a cap 8. The provision for the spring retainer is essential for an even distribution of the spring force on the diaphragm. As shown enlarged in FIG. 2, the diaphragm is integrally formed with an annular bead 9 serving as a compressible sealing portion of the diaphragm.
As is well-known, when the pressure in the sensing chamber is equal to an inlet pressure, the coiled spring biases the spring retainer which, in turn, urges the sealing bead 9 in contact with the valve seat so that the valve is closed. When the pressure differential developed across the diaphragm overcomes the spring force, the diaphragm is lifted away from the valve seat to permit the fluid flow.
Typically, the components parts of the diaphragm valve are manufactured by a mass production process. Unless subjected to precision machining, the plane of the valve seat as manufactured may suffer from a certain degree of unevenness or lack of flatness so that a circumferential undulation 10 may be present in the plane of the valve seat as shown in FIG. 3. Formation of undulation is particularly noticeable when the valve body 11 is manufactured by molding of a plastic material, since the moldings undergo an uneven shrinkage during a cooling process. Similarly, the lack of flatness may also be present in the spring retainer.
In order to establish a good sealing contact between the diaphragm and the valve seat in the closed position of the valve despite the presence of the foregoing undulation, it is desirable that the force of the coiled spring be strong enough to flatten and compress the sealing bead 9 against the valve seat 4 throughout the entire circumferential contour of the valve seat.
However, in certain applications, such as the diaphragm valve for use in controlling a flow of fuel vapor in an automotive onboard refueling vapor recovery (ORVR) system, it is desirable to design the valve in such a manner as to open and close in response to a small pressure differential. In such applications, a coiled spring must be used which only has a limited spring force.
With such a limited spring force, it is difficult for the coiled spring to tightly compress the sealing bead 9 against the valve seat throughout the entire circumference of the valve seat. As a result, the undulated valve seat or spring retainer would lead to a formation of a gap 12 left between the diaphragm and the valve seat, as shown in FIG. 3. This gives rise to a fluid leakage in the closed position of the valve.
Accordingly, it is the primary object of the present invention to provide an improved diaphragm valve of the class mentioned, which is able to completely shut-off the fluid flow in the closed position of the valve despite the presence of any undulation or lack of flatness of the valve seat and/or the spring retainer.
Another object of the invention is to provide an improved diaphragm valve which is operable in response to a small pressure differential and which is yet operable to shut-off the fluid flow with a high degree of sealability in the closed position of the valve.
As the diaphragm is subjected for a long period of time to an atmosphere containing a fuel vapor, the elastomeric material forming the diaphragm is swelled by the fuel vapor. This causes the diaphragm to expand so that the annular bead 9, which originally was in registration with the valve seat as shown in FIG. 4A, becomes offset radially outwardly away from the associated valve seat as shown in FIG. 4B. This also brings about an inadvertent fluid leakage.
Accordingly, a still another object of the invention is to provide an improved diaphragm valve which is capable of preventing the fluid leakage for a long period notwithstanding the occurrence of the swelling of the diaphragm.
In another form of the conventional diaphragm valves, such as the ones shown in FIG. 5A, an annular sealing bead 13 is formed on the valve seat. The foregoing problem of undulation is also encountered in these valves. In addition, while the valve is closed under the spring action, the annular bead 13 bites into the diaphragm so that a persistent deformation 14 may be formed on the diaphragm. When the diaphragm swells or the spring retainer is offset as shown in FIG. 5B, the sealing engagement between the valve seat and the diaphragm becomes unstable or insufficient so that there is a risk of fluid leakage.
Accordingly, a further object of the invention is to provide an improved diaphragm valve which is exempt from formation of a persistent deformation.